1. Field of the Invention
The present invention relates to a panel light source device and a display device, and more particularly to improvement on a panel light source device, which are used in a display device for presenting letters, an image and others thereon, for example, a liquid crystal display device, and on a display device including this panel light source device.
2. Description of the Related Art
A liquid crystal display device has features such as being slim, light, low in power consumption and others in comparison with other kinds of displays, and has been widely used as displays for use in various applications such as those starting with a computer, followed by portable terminals including a portable telephone, a car navigation terminal and others. In a case of a liquid crystal display device, since a liquid crystal panel on which image information on each pixel is reproduced does not emit light by itself, a light source device is required additionally, so that a liquid crystal display device is commonly equipped with a light source device called a back light device emitting light at a rear face side of a liquid crystal panel.
In such a display, since there is necessity that a display region for an image or the like is illuminated uniformly by light, there has been a demand for a panel light source device, having a light emitting surface corresponding to the display region, and being excellent in uniformity of a light emitting characteristic on the light emitting surface. Furthermore, a request has been made for downsizing and weight reduction of the light source device in order to realize downsizing and weight reduction of the light display device as a whole. For this reason, in this kind of a light source device, a side light type has been commonly adopted in which a linear shaped light emitter is disposed at the side-face side of a light guide plate.
FIG. 1 is a sectional view showing an example construction of a conventional panel light source device, showing a back light device for a liquid crystal display device adopting a side light type. In the figure, a numerical reference 1 indicates a light guide plate, 2 an optical sheet, 3 a light emitter, 4 a wire, 5 a reflecting sheet, 6 a front face case, 7 a rear face case and 12 an opening section. The constituents 1 to 5 are housed in a composite case formed by engaging the front face case 6 and the rear face case 7 with each other to construct a panel light source device and a liquid crystal panel is mounted outside the opening section 12 in a case of a liquid crystal display device.
In order to realize downsizing and weight reduction of the light source device, the reflecting sheet 5 is provided not only on the rear face side of the light guide plate 1, but also used as a lamp reflector obtained by bending an end portion into the shape of a Greek letter Π open sideways. Light emitted from the light emitter 3 strikes an light incident face provided at a side face of the light guide plate 1 to advance in the light guide plate 1 and exits from the front face (the optical sheet 2 side) thereof. The light leaving the front face is emitted through the opening section 12 through the optical sheet 2. That is, the opening section 12 works as a light emitting surface.
In a case where a clearance is present between the reflecting sheet 5 and the light guiding plate 1, since the emitting light from the light emitter 3 leaks through the clearance, part of the light is not used effectively, so that a luminance on the light emitting surface is reduced. Furthermore, light evaded from the clearance becomes stray light and the stray light appearing on the light emitting surface causes luminance irregularity of the light source device, having led, in turn, to a problem of display irregularity when such a light source device is adopted in a liquid crystal display device. Since this kind of display irregularity is extremely severe, a light source device with such a problem could not be used as a panel light source without an improvement thereon. Therefore, while a prior art light source device of this kind was provided with a print 13 for absorption of light on part of the optical sheet 2, the print 13 caused cost-up and further, became an factor of reduction in luminance.
Moreover, when an external force is imposed on the front face of the front face case 6, the front face case 6 is deformed to press the optical sheet 2 toward the light guide plate 1 side, so that a problem has arisen that a display quality is greatly degraded. Especially in a case of a liquid crystal display device, a liquid crystal panel is mounted onto the front face case 6, so that a force is imposed on the front case 6 with ease, which has adversely influenced a display quality.
FIG. 2 is a sectional view showing an example construction of another prior panel light source device, showing a device disclosed in Japanese Unexamined Patent Publication No. 2000-315414. In the figure, a numerical reference 1 indicate a light guide plate, 3 a light emitter, 5 a reflecting sheet, 8 a double-sided tape, 9 is a case and 10 a liquid crystal panel.
In this panel light source device, not only does an end portion of the reflecting sheet 5 overlap the front face of the light guide plate 1, but the overlapping portion thereof is firmly fixed to the front face of the light guide plate 1 with the double-sided tape 8. If a material strong in restoring force against bending is used as that of the reflecting sheet 5, it is not easy to suppressing occurrence of a clearance with the double-sided tape 8. In addition, the double-sided tape 8 adhered to the light guide plate 1 causes unexpected reflection in the light guide plate 1 to disable uniform luminance on the light emitting surface, having again resulted in a problem of reduction in luminance.
The light guide plate 1 total-reflects incident light based on a difference in refractive index between the material thereof (acrylic resin and the like) and air, not only to confine the light within the light guide 1 but also to scatter the confined light with a pattern formed on the rear face thereof and to thereby cause the confined light to leave from the front face. For this reason, reflection different from total reflection arises in part of the light guide plate 1 to the surface of which the double-sided tape 8 is adhered. Since light reflected at an angle different from the case of the total reflection is emitted from an unexpected location on the light guide plate 1, a problem has arisen that a kind of treatment (print 13) for absorbing the light is necessary to be applied on part of the optical sheet 2 similarly to the case in FIG. 1.
On the other hand, FIG. 3 is a sectional view showing an example construction of still another prior art panel light source device, showing a device disclosed in Japanese Unexamined Patent Publication No. 2001-250416. In the figure, a numerical reference 1 indicates a light guide plate, 3 a light emitter, 4 a wire, 5 a reflecting sheet, 6 a front face case, 7 a rear face case and 11, 14 a double-sided tape.
The double-sided tape 14 fixedly adheres the reflecting sheet 5 to the case 6 for the purpose of preventing dust from invading the interior of the optical source device from the external environment. For this reason, a problem has arisen that a clearance is easy to occur between the reflecting sheet 5 and the light guiding plate 1. Furthermore, since the optical sheet 2 is overlapped on the reflecting sheet 5, a force externally imposed on the case 6 is transmitted to the optical sheet 2 to degrade a display quality.
Since conventional panel light source devices were constructed as described above, a clearance between a reflecting sheet 5 and a light guide plate 1, if any, caused a problem of reduction in luminance on a light emitting surface or luminance irregularity on a light emitting surface, which required printing on a optical sheet 2 in order to suppress such adverse influences. In addition, when a force was imposed on a case, the case in turn pressed the optical sheet 2 to thereby, greatly degrade a display quality.